Studies on the biochemical and biophysical properties of nucleic acids and poly peptides of mammalian leukemia and sarcoma retroviruses have shown that: (1) The subunit RNA of sarcoma viruses examined is smaller than the subunit RNA of leukemia viruses. (2) Homopolyribonucleotide sequences (A,C,G) have been detected in the RNA of retroviruses from avian to primate isolates. (3) Poly(C) and poly(G) sequences, present in retrovirus RNA are transcribed into poly(dG) and poly(dC) sequences, respectively, in the retrovirus DNA. (4) The 3'-terminal poly(A) sequences of retrovirus RNA are not transcribed into poly(dT) sequences, in retrovirus cDNA. (5) Little or no poly(C) and poly(G) sequences have been detected in the nucleic acids examined from nonmalignant mammalian and human embryonic cells examined. (6) Genomic and subunit RNA of mammalian retroviruses have intrastrand and interstrand regions of double-stranded RNA.